Producing pulverulent iron for powder metallurgy by compacting feed stocks



United States Patent 3,418,104 PRODUCING PULVERULENT IRON FOR POWDER METALLURGY BY COMPACTING FEED STOCKS Harry R. Hatcher, Crown Point, Ind., assignor, by mesne assignments, to SCM Corporation, New York, N.Y., a

corporation of New York No Drawing. Filed Apr. 19, 1966, Ser. No. 543,557

14 Claims. (Cl. 7s-.s

The present invention relates to powder metallurgy and, more particularly, to a process for producing iron powder adaptable for powder metallurgy by compacting a feed stock prior to its decarb-urization and/or deoxidation.

Within recent years, the practice of fashioning machine parts or the like from metallic iron powder has undergone significant growth. Articles produced by powder metallurgy commonly replace articles of cast iron or steel formed by the more conventional casting or machining operations. For example, compactible iron powder is used for the fabrication of a wide variety of structural parts, such as sleeves, bushings, gears, etc. For this purpose, the iron powder must possess a wide range of acceptable physical properties, including acceptable values for apparent density, fiow, green strength, tensile strength and modulus of rupture after sintering, and still other certain physical characteristics. H

In general, to produce an iron powder adapted for use in powder metallurgy a suitable high carbon iron, either alone or mixed with elemental (free) iron, is comminuted to define a powder feed stock which is then decarburized as by chemical oxidation. For instance, a powder feed stock of high carbon iron, and iron and an oxide of iron is treated under such conditions that the carbon in the iron combines with the oxygen of the iron oxide to reduce the latter to iron, while the high carbon iron is simultaneously decarburized. The resulting iron powder is then compacted in a mold into substantially the shape and size desired in a finished product, in this way dispensing with expensive machining operations. After compaction, which may be at a pressure up to 100,000 pounds per square inch, the resulting shape is heated at any exemplary temperature of about 2,000 F. to about 2,100 F. under non-oxidizing conditions, for a sufiicient time to produce the ultimate sintered article.

Normally, one would expect that compacting powder fed stock prior to its decarburization and/or deoxidation (hereinafter generically referred to as reduction) would introduce serious problems and, at a minim-um, result in a less satisfactory pulvenulent iron powder for powder metallurgy applications. For example, the standard procedure is to grind a feed stock to a certain loose particle size prior to reduction, rather than to do the opposite and compact it, to aggregates. Further, compacting a feed stock would logically seem to retard its reduction in view of the greater difficulty of a reducing gas or the like to permeate the densified feed stock. Still further, it is well established in the art that the finer the particle size of the powder feed stock, the more efficient and faster is a reduction reaction. In short, the art at present definitely teaches an opposite practice, that is, one way from compacting the feed tsock prior to its reduction.

Contrary to all expectations, it has been discovered that compacting a feed stock prior to the subsequent steps of reduction, molding, and sintering results in unforeseen advantages in various steps of the overall process. For example, a compacting or briquetting step heats the particles so that they become ostensibly dry. The close intimacy of the compacted particles actually leads to better heat transfer at a faster rate of reduction.

Another discovered important advantage of the present process is that extremely fine particle size may 3,418,104 Patented Dec. 24, 1968 ice be used to realize efficient reduction, and yet there is achieved effective agglomeration of the reduced product to a grade of coarseness which is desirable for the powder metallurgy art. For instance, the particulate reduced iron when used for powder metallurgy should flow well and completely fill molds easily. The reduced powder of the present invention not only can be used effectively as just described, but also the product has good green strength.

A still further advantage of the present invention is that the reduction and agglomeration within a reducing zone can be accomplished at a lower temperature to yield substantially the same particulate size, green strength, sinter strength, and flow properties as attained at a higher temperature with a feed stock which has not been precompacted. Correlated with this advantage is that precompaction followed by reduction yields a powder that does not have to be extensively mechanically treated after reduction in order to have acceptable molding properties. As a consequence, work-hardening of the powder, which may result form such mechanical treatment, is avoided. This, in turn, eliminates a heat treatment normally necessary to remove any work-hardening effect from the powder.

The present process thus yields an aggregate or briquet which reduces easily as though it were a fine powder, and yet which also yields in iron powder that has good green strength coupled with 'a sufficiently .high bulk density to pour and fill molds without a post-processing ag-glomerating treatment that normally tends to workharden the powder. In brief, pre-briquetting or precompaction has been found to aid rather than adversely affect reduction, to eliminate after treatments, and to provide good green strength.

More particularly, the present invention relates to a process for producing pulverulent iron adaptable for powder metallurgy by reducing a particulate iron oxidecontaining material with a reducing agent in a reaction zone having a non-oxidizing atmosphere, wherein such material is compacted at a stage before it is reduced to substantially pure iron. Normally thte iron-oxide containing material or feed stock is compacted prior to any reduction, although it is possible to compact the feed at some intermediate stage of the reduction, as hereinafter more fully described. The feed stock is usually compacted at least one-quarter of its original bulk density, and preferably from about one-quarter to about five-eighths of the original bulk density. After compaction, the feed stock is passed into a reaction zone, either for the first time or, as indicated, to continuethe reduction.

Generally, the feed stock comprises a mixture of iron, iron carbide, and a metal oxide, such as iron oxide, capable of reacting with the iron carbide to yield elemental metal. A preferred component furnishing the iron and iron carbide is a hypereutectic composition consisting essentially of coarse grain primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic which is preferably free of graphite and contains about 4.1 percent to about 4.6 percent carbon. The intrusions of platelets of iron carbide into the matrix (which can be observed under a microscope) contribute materially to the grindability or frangibility of the eutectic. This is important to easily powdering the composition. The amount of carbon present can also be important. If less than about 4.1 percent carbon is available, an insufficient number of the platelets is formed which deleteriously affects the desired frangibility. On the other hand, if more than about 4.6 percent carbon is present, graphite is apt to form, as during quenching of the melt which produces the hypereutectic composition. A small quantity of graphite can be tolerated, but for best results none should be present. Graphite tends to nucleate and form still more graphite.

A convenient source of the present hypereutectic composition is what is known in the art as white cast iron. This type of iron can be produced by rapidly quenching a melt of a high carbon-iron alloy. A brittle structure results that is low in austenite and high in cementite, pearlite, and martensite. The brittleness also contributes to the desired frangibility of the composition. In particular, Sorelmetal admirably provides a source of the present hypereutectic composition. Sorelmetal, a product of Quebec Iron & Titanium Corporation of Sorel, Canada, is a cast iron obtained from the electric arc furnace smelting of iron-bearing titaniferous ore, ilmenite, in the presence of coal.

An iron oxide-containing ferruginous material is added to and thoroughly mixed with the hypereutectic composition to reduce the iron carbide and simultaneously provide additional free metallic iron from the oxide. Any iron oxide can be used, such as naturally occurring ores like magnetite, hematite, and limonite. Similarly, partially reduced iron oxides having about two percent to about 30 percent hydrogen loss (as determined by conventional M.P.I.F. test) can also be used. A quite inexpensive and readily available source of an iron oxide is mill scale which comprises mixed iron oxides containing about 75 percent iron. The relative amounts of the iron carbide and iron oxide in the free iron mix are not at all critical as long as at least sufficient oxide is present stoichiometrically to remove substantially all of the carbon. As previously indicated, complete removal of the carbon is not essential as when the presence of a little (less than about 1.5 percent) can be tolerated. In practice, from about 60 to 85 parts by weight of a Sorelmetal have been mixed with from about to about 40 parts by weight of mill scale. A ratio of about 80 parts of Sorelmetal to about parts of mill scale is preferred.

In the usual procedure, the feed stock is comminuted to powder form by known conventional means. Compaction of the powder feed stock likewise is affected by standard machinery, such as a briquetting machine, to form discrete aggregates or briquets, although it is understood that some fines may still remain in the feed stock. Ordinarily, about 80 percent by weight of the compacted feed stock is coarser than 4 mesh, and about 20 percent by weight is finer than 4 mesh. All references here and in the claims to a mesh size are with respect to the Tyler Scale. The powder feed stock (prior to any reduction treatment) has a bulk density up to about three grams per cubic centimeter. As indicated, the feed stock is compacted by a factor of at least one-quarter of its original bulk density, and preferably from about one-quarter to about five-eighths of such density. The discrete aggregates formed have a density greater than about 3.8 grams per cubic centimeter. In the case where the feed stock includes Sorelmetal, optimum results have been obtained when the compacted aggregates had a density of at least about 4.5 grams per cubic centimeter, although this is not a critical minimum. Within practical and economic limits, there is little or no risk of over-campaction.

To enhance the advantages afforded by compaction in accordance with the present invention and to impart adhesiveness to the compacted aggregate, a fugitive binding agent may be dispersed in the powder feed stock. Pref-- erably the binding agent is fugitive so as effectively to disappear during the chemical reduction step. For the same reasons, the binding agent should be substantially ashless. Water has been found to serve these purposes quite well and may be used in an amount up to about four percent by weight of the compacted feed stock.

The compacted feed stock is next reduced. This step can be carried out conventionally in a single stage or a plurality of stages. Two reduction stages are preferred for efiiciency and economy of operation. A single reduction stage or the first reduction stage, in which the bulk of the reduction may be done, can be operated at a temperature Ill) of about 1600 F. to 2000 F. with elfectiveness and preferably is operated at about 1700 F. to 1750 F. for efiiciency and economy. The second stage, in which only a small amount of finishing reduction may be done, can be operated at an even higher temperature, if desired, without danger of over-sintering or liquefying the particles. The second stage treatment can be effective at temperatures as low as about 1350 F., although the most efficient temperature is about 1700" F. to 1750 F.

The atmosphere of the reduction should be neutral to the material in process (e.g., preponderantly nitrogen, argon, helium, or the like) or preferably reducing to give more efi'iciency and flexibility to the operation (pure hydrogen, gen, and dissociated ammonia). The reduction preferably is. carried out in the substantial absence of molecular oxygen or materials capable of forming molecular oxygen, carbon dioxide, water vapor, or other gases capable of oxidizing iron under normal operating conditions. A typi* cal gas feed to supply a good atmosphere for the operation is volume percent hydrogen, and 25 volume percent nitrogen having a dewpoint of 39 F.

The preferred apparatus for reduction is a' thin layer continuous belt furnace, although the reduction can be performed batch-wise or in trays or saggers passed through a tunnel kiln. A slight positive pressure generally is main tained during reduction to prevent air from seeping into the apparatus. If the apparatus can be made tight, a high vacuum can be used, the rarefied atmosphere thus provided being suitable in itself as a neutral atmosphere when venting of gaseous by-products is practiced.

While it is not intended to restrict the invention in any manner by theory, it is postulated that at least the following four different reduction reactions may take place in the reducing environment described.

(I) Fe C/Fe O reductions.

(2) Fe O /carbon reduction.

(3) Fe O /reducing gas/ carbon reductions.

(4) Fe O /reducing gas, such as hydrogen or CO re duction.

In these reactions the subscripts x and y are taken to mean any oxide of iron, such as FeO, Fe O Fe O and/ or mixtures thereof. While Reaction 4 is clearly a gaseous reaction, Reactions 1 through 3 may'possibly be either a gaseous or solid reaction, or a combination thereof. While the following concept is only theory at present, one possible explanation of why good chemical reduction occurs in spite of compacting the powder feed stock is that the carbon monoxide generated during reduction Within the aggregate migrates outwardly under its own self-generating pressure, even though the permeability of the aggregate is less than it would be in an uncompacted feed stock. Also, the oxides to be reduced are present in a much more concentrated volume.

For instance, when Fe C is heated, it decomposes to form some free carbon which combines with oxygen to form carbon monoxide. This in turn acts upon an oxide of iron to reduce it to free iron. In the case of Reaction 2, again the reducing agent is probably carbon monoxide. The same theory applies to the remaining two listed reactions.

The following examples only illustrate the invention and should not be construed in any manner to limit the claims. All percentages indicated are by weight. In the case of screen analysis, the percentage listed represents the amount by weight passing through the indicated screen size on the Tyler Scale. 7

Example 1 hydrogen containing small proportions of nitro- TABLE A Sorelmetal Mill scale Chemical anlysis:

Carbon 4.2-4-4 0.30 max. Sulfur 0.925 max 0.05 max. Phosphorus" 03 max. Coppen Silicon 0.08 ma 0.10 max. Manganese 0.20 max. Titanium. 0.15 max Hydrogen Loss 25.0 max. Acid Insolubles 0.35 max 0.20 max. Moisture 3-4 4.0. Screen analysis:

On 3 mesh..- 5.0 max On 4 mesh10.0 max. On 4 mesh. 10.0 max On mesh515. On 8 mesh.-- 30.0 max On 20 mesh--25. On 14 mesh 50 0 max. On 35 mesh-35. On 28 mesh 5-30 On 48 mesh-10-20. Thru 28 mesh 5-25 On 60 mesh-3-10.

Thru 60 mesh-10-30.

Determined in accordance with M.P.I.F. Std. 2-64, issued 1948,

revised 1964.

Z Determined in accordance with M.P.I.F. Std. 6-64, issued 1954,

revised 1964.

Each component was separately dried and then separately ground in a' ball mill an dscreened. In each case, grinding was continued until about 72 percent to about 80 percent passed 325 mesh, and about 0.2 percent to about 1.0 percent was retained on 150 mesh. The advantages accruing from grinding the feed stock to a certain particle size form the subject matter of another application, filed of even date herewith in the names of Harry R. Hatcher and William Holtzman, entitled, Iron Powder for Forming Sintered Articles of Improved Strength, and assigned Ser. No. 543,570.

The Sorelmetal and mill scale were next thoroughly and intimately mixed together in a weight ratio of 4:1. The mixture had a bulk density of 2.9 grams per cubic centimeter. Water was admixed in an amount of about three to about four percent of the powder feed stock, and this stock was then compacted by a roll briquetter having a roll pressure of about 2200 to about 2400 pounds. Briquets were yielded measuring 1% x A x inches. The rolls of the briquetter rotated at about 6 r.p.m.s, and were spaced about inch apart to keep fines at a miniphere' so that it passed an 80 mesh The intermediate product was then sent through a second thin layer furnace maintained at 1750 F. The ad vantages accruing from a plurality of reduction passes form the subject matter of another application, filed of even date herewith in the names of Harry R. Hatcher and Charles R. Harr, and assigned Ser. No. 543,545. The belt speed in the second furnace was about six inches per minute, and the bed depth was about 1% inches to yield a holding time at the temperature stated of about 20 to 25 minutes. Particle feed was continuous at the rate of about 1200 pounds per hour. The product output discharged from the furnace continuously at about 1160 pounds per hour. The product was ground in inert atmosscreen and had the following properties: 0.03 percent carbon; a hydrogen loss of 0.08 maximum; apparent density of 2.25 to 2.50 grams per cubic centimeter; and a pressed density at 30 T.S.I. of 6.5. A maximum amount of 35 percent of the particles were finer than 325 mesh. The resultant iron powder product could then be used to form shaped articles by standard pressing and sintering operations.

Example 2 Two powder feed stocks were prepared having the following composition:

Feed Stock 1: 80 parts by weight of Sorelmetal to 20 parts by weight of mill scale, these components having 85 percent and 84 percent of fines, respectively.

Feed Stock 2: This composition was similar to Feed Stock 1 except that the mill scale contained only 12 percent fines.

briquetted material was coarser than four mesh and 20 percent was finer than four mesh. On the basis of the 2.9 bulk density of the feed powder to the briquetter,

different runs resulted in densification of about 38 percent, about 36.5 percent, and about percent, resulting, respectively, in a briquet density of 4, 4.25, and 4.5.

The briquets were reduced in a thin layer continuous belt furnace at a temperature of 1750 F. in an atmosphere of dissociated ammonia. The bed of the furnace was a moving belt travelingat 5% inches per minute to yield a reduction time at the temperature stated of about 10 minutes. The approximate feed rate in pounds per hour was 1325. The 'bed depth of briquets being reduced was about 1.5 inches. The intermediate product emerged continuously from the furnace at an approximate rate of 1165 pounds per hour, having a hydrogen loss of 1.50 maximum and carbon content of 0.30 percent maximum. This intermediate product was ground in an attrition mill in an atmosphere of inert gasto particles passing mesh and having a density between about 2.2 and 2.4 grams per cubic centimeter.

The green strength value was determined by the Standard Test 16-61 of the Metal Powder Industries Federation. The data of Table A show green strengths ranging from about 2345 p.s.i. to about 2600 p.s.i. This compares quite favorably with competitive commercial powders which have green strengths of about 1800 p.s.i. to about 2200 p.s.i. under identical test procedures. The values for the green strengths were obtained by the M.P.I.F. Standard Test 15-61. Similarly, the Hall Flow value, which is a measure of flow of the powder through agiven orifice, was determined by M.P.I.F. Standard Test 3-45. The symbol TSI means tons per square inch.

Example 3 This example compares the effect of compaction of the feed stock as against no compaction. Two additional feed stocks were prepared having the following compositions:

Feed Stock 3: 80 parts by weight of Sorelmetal to 20 parts by weight of mill scale, the Sorelmetal being ground to about 50-55 percent through 325 mesh, and the mill scale being ground to about -80 percent through 325 mesh.

Feed Stock 4: This composition was similar to Feed Stock 3, except that the mill scale was coarser, only 36.4 percent passing 325 mesh.

TABLE 13 Feed stock Pressed density Green strength at 30 t.s.i. at 30 t.s.i.

3No compaction. 6. 59 i l, 571 3--Compacted 6. 36 2, 100 4-No compaction 6. 56 1, 237 4-Compacted... 6. 39 2, 105

Example 4 Compaction of the feed stock in this instance took place prior to the first pass, that is, before any reduction. The feed stock consisted essentially of 80 percent by weight of Sorelmetal and 20 percent by weight of mill scale, each ground to about 72 percent by weight passing 250 mesh. Water in an amount of about four percent was added, and the resulting mixture compacted by briquetting to a density of about 4.5 grams per cubic centimeter. The compacted powder was then treated in accordance with the process of Example 1, with the stock being attrition milled after the first pass to continue to the second pass only that powder passing 35 mesh. The resulting powder after the second pass had these properties, the values being obtained in the manner previously described.

(A) A green strength of 3180 p.s.i. (at 30 T.S.I.) (B) A pressed density of 6.50 (at 30 T.S.I.)

(C) An apparent density of 2.47

(D) A Hall Flow Test of 29.5 seconds (E) A fines content of 30.2%

While the foregoing describes several embodiments of the invention, it is understood that the invention may be practiced in still other forms within the scope of the following claims.

What is claimed is:

1. In a process for producing pulverulent iron adapted for powder metallurgy by reducing a powder feed stock comprising a mixture of iron, iron carbide, and iron oxide in a reaction zone having a non-oxidizin atmosphere, the improvement which comprises: compacting said powder feed stock by a factor of at least about one-quarter of its original bulk density, and subjecting the resulting compacted feed stock to said reaction zone.

2. The process of claim 1 wherein said iron carbide is a hypereutectic composition consisting essentially of primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic.

3. The process of claim 2 wherein said hypereutectic composition is substantially free of graphite and contains about 4.1 percent to about 4.6 percent by weight of carbon.

4. The process of claim 3 wherein said feed stock includes Sorelmetal.

5. The process of claim 1 wherein said iron oxide comprises mill scale.

6. The process of claim 2 wherein said feed stock includes Sorelmetal, and such compaction produces discrete aggregates having a density of at least 4.5 grams per cubic centimeter.

7. The process of claim 1 wherein said powder feed stock prior to said compaction has a bulk density up to about 3 grams per cubic centimeter, and compaction produces discrete aggregates having a density greater than about 3.8 grams per cubic centimeter.

8. The process of claim 1 wherein said atmosphere is a reducing atmosphere.

9. The process of claim 1 wherein said atmosphere consists essentially of hydrogen.

10. The process of claim 1 wherein said compaction of the powder feed stock is by a factor within the range of about one-quarter to about five-eighths ofthe original bulk density.

11. The process of claim 1 wherein said powder feed stock further contains a dispersed fugitive binding agent.

12. The process of claim 11 wherein said binding agent is substantially ashless, and the compaction produces dis crete aggregates.

13. The process of claim 11 wherein said binding agent is water present in an amount up to about four percent by weight of the compacted feed stock.

14. The process of claim 1 wherein about 80 percent by weight of the compacted feed stock is coarser than four mesh, and about 20 percent by weight is finer than four mesh.

References Cited UNITED STATES PATENTS 3,073,695 1/1963 Silbereisen et a1 -211 3,194,658 7/1965 Storcheim 75-211 3,214,262 10/ 1965 Von Bogdandy et a1. 75-211 3,326,676 6/ 1967 Riibel et al 75201 3,368,890 2/ 1968 Schroeder et a1. 75 0.5

L. DEWAYNE RUTLEDGE, Primary Examiner. W. W. STALLARD, Assistant Examiner.

US. Cl. X.R. 

1. IN A PROCESS FOR PRODUCING PULVERULENT IRON ADAPTED FOR POWDER METALLURGY BY REDUCING A POWDER FEED STOCK COMPRISING A MIXTURE OF IRON, IRON CARRBIDE, AND IRON OXIDE IN A REACTION ZONE HAVING A NON-OXIDIZING ATMOSPHERE, THE IMPROVEMENT WHICH COMPRISES: COMPACTING SAID POWDER FEED STOCK BY A FACTOR OF AT LEAST ABOUT ONE-QUARTER OF ITS ORGINIAL BULK DENSITY, AND SUBJECTING THE RESULTING COMPACTED FEED STOCK TO SAID REACTION ZONE. 